Hydroformylation is a well known reaction in which an olefin, usually a terminal olefin, is reacted under suitable temperature and pressure conditions with hydrogen and carbon monoxide in the presence of a hydroformylation catalyst to give an aldehyde, or a mixture of aldehydes, having one more carbon atom than the starting olefin. Thus a hydroformylation reaction with propylene will yield a mixture of n- and iso-butyraldehydes, of which the straight chain n-isomer is usually the more commercially desirable material. The hydrogen and carbon monoxide will generally be supplied to the hydroformylation reactor as synthesis gas.
Examples of hydroformylation processes can be found in U.S. Pat. Nos. 4,482,749, 4,496,768 and 4,496,769 which are incorporated herein by reference.
The catalysts first used in hydroformylation reactions were cobalt-containing catalysts, such as cobalt octacarbonyl. However, the presence of these catalysts meant that the reactor had to be operated at exceptionally high pressures, e.g. several hundred bars, in order to maintain the catalysts in their active form.
Rhodium complex catalysts are now conventionally used in the hydroformylation of both internal olefins and alpha-olefins, that is to say compounds containing the group —CH═CH2, —CH═CH—, >C═C<, >C═CH—, —CH═C< or >C═C2H. One advantage of these catalysts is that lower operating pressures, e.g. to about 20 kg/cm2 absolute (19.6 bar) or less, may be used than was usable with the cobalt catalysts. A further advantage noted for the rhodium catalysts was that they are capable of yielding high n-/iso-aldehyde product ratios from alpha-olefins; in many cases n-/iso-aldehyde molar ratios of 10:1 and higher can be achieved.
Further, since the rhodium catalyst is non-volatile, product recovery was greatly simplified. A fuller description of the process can be found in the article “Low-pressure OXO process yields a better product mix”, Chemical Engineering, Dec. 5, 1977. Also relevant to this process are U.S. Pat. No. 3,527,809, GB-A-1338237 and GB-A-1582010 which are incorporated herein by reference.
The rhodium catalyst generally adopted in commercial practice comprises rhodium in complex combination with carbon monoxide and with an organo-phosphorous ligand, for example triphenylphosphine. Although the nature of the catalytic species is not entirely clear, it has been postulated that where the ligand is triphenylphosphine it is HRh(CO)(PPh3)3 (see, for example, page 792 of “Advanced Inorganic Chemistry” (Third Edition) by F. Albert Cotton and Geoffrey Wilkinson, published by Interscience Publishers).
The reaction solution for the hydroformylation reaction will generally contain excess ligand.
U.S. Pat. No. 3,527,809, which is incorporated herein by reference, proposes the use of other ligands, including phosphites, such as triphenylphosphite.
Whilst the use of rhodium catalysts offers various advantages, it does suffer from the disadvantage that it is very expensive. It is therefore desirable to utilise this highly expensive metal in the most economically effective way.
During operation of the reactor, the catalyst may become deactivated and therefore needs to be removed from the reactor such that fresh active catalyst can be added. The removed catalyst will generally be processed to recover the metal values.
The deactivated catalyst may have been thermally deactivated, i.e. clustered and/or chemically deactivated, i.e. poisoned or inhibited.
In some cases although the catalyst may be chemically active, the catalyst solution includes such a high concentration of non-volatile material that it is of no further practical use.
Although the mechanism of deactivation in aryl phosphine liganded systems by the formation of clusters is not entirely clear, it is believed that rhodium clusters, having phosphido bridges may be formed, for example, by the loss of one or more phenyl groups from the aryl phosphine molecule. The formation of clusters is generally increased as the temperature is increased.
The chemical deactivation may be poisoning such as by sulphur compounds, chloride, cyanide and the like.
The chemical deactivation may also be inhibition of the catalyst. Inhibitors that may be found in, for example, propylene and butylene hydroformylation include acetylenes and acroleins.
Since the rhodium catalyst is generally used in low concentration because of its high cost and activity, the effect of any poisons or inhibitors present is high. It is therefore usually necessary to reduce the presence of these poisons and inhibitors present in the feed to very low levels.
Rhodium catalysed hydroformylation processes can be classified into two main categories, namely those in which the aldehyde product is removed by liquid/liquid separation processes and those in which the product is removed by a vapour path process.
In the processes in which the aldehyde product is removed by a liquid/liquid separation process, the aldehyde product is obtained as one liquid phase while the ligand and rhodium/ligand complex remains in another phase and is returned to the reaction zone. This type of process has the advantage of being independent of the volatility of the aldehyde product and the volatility of the relatively less volatile aldehyde condensation by-products. These processes do, however, have their own disadvantages including interphase solubility/entrainment problems in which some of the rhodium may leave in the aldehyde product-containing phase, low selectivity to the desired aldehyde isomer and low reaction rate as a consequence of the low solubility of the reactants in an aqueous base reaction medium.
Where the aldehyde product is recovered from the catalyst by a vapour path process this has conventionally been effected in one of two ways.
Where lower olefin feedstocks are used, a stream of synthesis gas and olefin is passed through the reactor solution, condensed and after separation of the liquid condensate the gas phase is returned to the reactor via a compressor. Suitable means is used to prevent the rhodium solution leaving the reactor by liquid droplet entrainment in the gas phase these include restricting the superficial velocity of the gas through the reactor to less than a specific value and passing the gas/vapour stream through a liquid droplet de-entrainment device before exiting the reactor. Addition of make-up streams of synthesis gas and olefin are required to maintain the system pressure and reaction rate as the reactants are consumed. A purge stream of gas after the condensation stage is generally required to remove any inert gases accumulating in the system and also to control the level of paraffins that either enter the system with the olefin feed or are produced by olefin hydrogenation in the reactor. This type of process is generally known as a Gas Recycle Process.
An important feature of the Gas Recycle Process is that to achieve stable reactor conditions, every product of the reaction must leave the reaction system at it's rate of formation, thus the relatively less volatile materials (such as aldehyde condensation products) accumulate in the reactor solution to a relatively high concentration until the rate of removal of products in the vapour phase equals the production rate of each material. This can be achieved for long periods when the feed olefin is ethylene or propylene but even with propylene there can be a slow accumulation of aldehyde condensation tetramers and pentamers such that the reactor solution volume will slowly increase with time.
If progressively higher olefins such as butenes, pentenes, hexenes etc. are supplied to a gas recycle system the requirement for a higher gas recycle rate means that the gas superficial velocity limit is exceeded unless a reactor solution volume that is increasingly wide and shallow is used as the olefin molecular weight increases. Thus, whilst this arrangement goes some way towards addressing the problems detailed above, the arrangement suffers from new problems associated with gas/liquid mass transfer and reactor mechanical/economic design issues.
In an alternative solution to the problem associated with the use of higher olefins, the temperature of the reaction system is increased such that every component becomes more volatile. Again, whilst this arrangement goes some way to solving the above problem, fresh problems are noted. In this case, increased production of heavy aldehyde self condensation by-products and increased catalyst deactivation by increased clustering rates occurs.
These considerations mean that the Gas Recycle Process is limited to the hydroformylation of ethylene and propylene with the hydroformylation of butenes and pentenes being marginal and very marginal cases respectively.
These considerations led to the development of the so called “Liquid Recycle Process”. In this process a volume of solution is continuously withdrawn from the hydroformylation reaction zone or zones such that the liquid level in the or each zone is held constant. This withdrawn liquid is then subjected to a single or multistage evaporation operation where the temperature, pressure and residence times are selected to recover the products and by-products as well as to protect the catalyst activity. The concentrated catalyst solution is then returned to the hydroformylation reaction zone. Olefin and synthesis gas are supplied to the or each hydroformylation reaction zone to maintain the desired reaction rate and conditions.
The liquid recycle process has been shown to provide benefits even for the hydroformylation of propylene where higher volumetric productivity and lower operating costs can be achieved, and is essential for the economic production of C5 and higher aldehydes.
As olefins of increasing molecular weight are hydroformylated by the Liquid Recycle Process the removal of the heavy by-products by evaporation requires lower and lower pressures and/or higher evaporation temperatures. Thus, despite the advantages noted for this process, eventually the accumulation of heavy by-products in the reactor solution occurs such that the reactor volume increases uncontrollably. This disadvantage of the system is referred to as “heavies drowning”. Where heavies drowning occurs, there has to be a purge of catalyst solution (containing ligand and active catalyst) to control this accumulation.
It has been suggested, for example in U.S. Pat. No. 5,053,551, that the addition of inert diluents can delay heavies accumulation to defer the heavies drowning effect and confer a longer useful catalyst life. Whilst the system goes some way to addressing the problem, it cannot prevent eventual heavies drowning from occurring.
Thus during the operation of a liquid recycle hydroformylation plant the reaction and product recovery conditions are in a state of continuous change due to the changes in solution composition and catalytic activity. The accumulation of essentially non volatile aldehyde condensation products requires that the pressure and/or temperature of the product evaporator needs progressive adjustment. The accumulation of inhibitors and poisons in the reactor solution also requires the progressive adjustment of reaction conditions to maintain the conversion and selectivity of the system. High temperature evaporation and poisons in the olefin feed can also result in the loss of catalytically active rhodium by poisoning and/or the formation of rhodium clusters requiring the continuous or periodic removal of a part of the catalyst recycle stream and its replacement by fresh catalyst and ligand.
Thus, it will be understood that whichever hydroformylation method is selected, the economic need to run the plant for maximum production of product must be balanced with the need to conserve the life of the expensive catalyst. It is therefore desirable to adopt catalyst management systems which maximise productivity whilst minimising the damage to the catalyst.
One catalyst management system which may be adopted comprises charging a first charge of catalyst to the plant. As the productivity of the plant begins to decline it is necessary to adapt the utilities and separation units of the plant to the reduced flow of aldehyde and the reduced consumption of synthesis gas. Care is taken to ensure that the temperature does not increase since any such increase will result in an accelerated decline in the catalyst activity and increased formation of the heavies. When product flow falls to a level that is unacceptable, the plant operator may choose to raise the temperature with the attendant problems or add additional catalyst.
Although increasing temperature does have the drawbacks detailed above it does not incur the capital expenditure of catalyst purchase and may therefore be the preferred initial approach. Any step change in the temperature will require a corresponding step change in the operation of the utilities and separation units.
After any increase in temperature the productivity will continue to decline but at an increased rate. Further increases in temperature may be carried out until a decision is made that any further increase will result in an unacceptable rate of catalyst deactivation. At this point further catalyst may be added to the reactor. However, increasing the catalyst concentration will also increase the rate of thermal deactivation and the consequential loss of activity. Thus there is an upper practical limit on the amount of rhodium which may be added to the reactor. Eventually it will be necessary to shut down the plant.
One alternative catalyst management system involves taking a continuous purge of the reactor solution which can then be reprocessed to recover the catalyst and remove the heavies. In practice, economics require that the catalyst be reprocessed in large batches and results in significant loss of rhodium metal. This results in high capital expenditure for the plant owners.
Where triphenylphosphine is used as ligand, it may react with the olefin to produce the corresponding alkyldiphenylphosphine. Since the alkyldiphenylphosphines are stronger complexing agents than the triphenylphosphine, a catalyst solution of lower activity and selectivity to the linear product is obtained.
These mechanisms of catalyst degradation become progressively more onerous as the molecular weight of the olefin increases, requiring progressively higher catalyst purge rates.
Conventionally, the operators of the gas or liquid recycle plant have had to collect the active and/or inactive catalyst by shutting down the reactor, removing some or all of the catalyst solution and concentrating it to partially separate it from the other components present. Additionally, or alternatively, partially deactivated or heavies drowned catalyst may be continuously collected from reactor streams. By reactor stream we mean any stream which is obtained from any point in a process and which will contain rhodium metal catalyst. In the case of the liquid recycle process, the stream will usually be the catalyst recycle stream after evaporation of the hydroformylation products.
The conventional liquid recycle process must therefore be subjected to a continuous or episodic regime of adjustment in process conditions throughout the operating period and this is particularly marked when the higher molecular weight olefins are used as feedstock.
Since the rhodium is generally only present at low concentration, it can be particularly difficult and costly to recover the rhodium from the very dilute solutions.
The rhodium organic solution has conventionally been concentrated by a variety of means before being shipped off-site for recovery. This means that if the operation of the plant is not to be shut down for a prolonged period, the operator must purchase more of the very expensive catalyst to operate the plant than he actually requires at any one time.
There are also environmental issues associated with the recovery of the catalyst where phosphorous ligands are present.
A variety of means of recovering the rhodium from solution has been suggested including precipitation followed by extraction or filtration and extraction from the organic mixtures using, for example, amine solutions, acetic acid, or organophosphines.
Ion-exchange methods have also been suggested, for example in U.S. Pat. No. 3,755,393 which describes passing a hydroformylation mixture through a basic ion-exchange resin to recover rhodium. A similar process is described in U.S. Pat. No. 4,388,279 in which Group VIII metals are recovered from organic solution using either a solid absorbent such as calcium sulfate, an anionic ion-exchange resin or molecular sieves.
An alternative arrangement is described in U.S. Pat. No. 5,208,194 in which a process is described for removing Group VIII metals from organic solutions which comprises contacting the organic solution with an acidic ion-exchange resin containing sulfonic acid groups. The treated solution is then separated from the ion-exchange resin and the metal values are recovered from the resin by any suitable means. One means that is suggested is that the resin should be burnt off in an ashing process which leaves the metal in a form suitable for recovery.
These prior art processes, whilst being suitable for separating the metal from the stream in which it was removed from the reaction, suffer from the disadvantage that the operator of the reactor must send the recovered metal concentrate off-site to be converted into an active form. Further, where the stream removed from the reactor includes active catalyst, the separation procedure will either leave it in a form in which it cannot be returned to the reactor or will cause it to be deactivated such that it is no longer suitable for use in the reactor and removal off-site for regeneration is required.
In U.S. Pat. No. 5,773,665, a process is suggested which enables active catalyst contained in a stream removed from a hydroformylation process to be separated from the inactive catalyst and the active catalyst following treatment, to be returned to the hydroformylation reactor. In the process a portion of the recycle stream from the hydroformylation reaction is passed through an ion exchange resin column to remove impurities and active rhodium and the thus purified recycled stream, which may contain inactive catalyst, is returned to the hydroformylation reactor.
The impurities, which may include aryl phosphine oxide, alkyl phosphine oxide, mixed phosphine oxide and high molecular weight organic compounds, are removed from the resin by washing with, for example, an organic solvent. The effluent from this wash is removed as a waste stream. The active catalyst remains bound to the resin during this washing process.
The resin is then treated with a catalyst removal solvent such as isopropanol/HCl to produce a stream containing “active” rhodium catalyst for eventual recycling to the hydroformylation reactor. Whilst the catalyst has not been deactivated by thermal or chemical means and is therefore referred to as “active” it is not in a form in which it will actually act as a catalyst in the reactor. Thus, before the catalyst can be recycled it must first be removed from the resin using a strong acid reagent and then converted to the hydridocarbonyl by treatment with hydrogen and carbon monoxide in the presence of an acid scavenger and a ligand to make it a truly active catalyst.
In an optional arrangement, the inactive rhodium catalyst, i.e. the clustered catalyst, which passed through the ion-exchange resin without being absorbed and which is contained in the purified recycle stream may be reactivated by conventional technology such as by wiped film evaporation followed by oxidation and subsequent reduction before being returned to the reactor. Thus this inactive catalyst is not treated by the ion-exchange resin.
Whilst this process goes some way to improving the conventional hydroformylation process by recycling some of the rhodium, in that it suggests a means of separating the active catalyst on site, it suffers from various disadvantages and drawbacks in particular those disadvantages associated with the need to treat the “active” catalyst after it has been removed from the ion-exchange resin and before it can be returned to the reactor. Indeed it is the ion-exchange treatment which means that the catalyst is no longer suitable for use in the reactor. Although in a preferred embodiment, U.S. Pat. No. 5,773,665 does suggest that the thermally deactivated catalyst may be regenerated before return to the reactor, the overall plant described therein is expensive to construct and operate because of the number of separation and treatment steps required to achieve full recycle. The problem is particularly exacerbated as some of the steps are carried out in the presence of corrosive acid media A further drawback associated with the presence of acid media is the costs associated with the consumption of base required to neutralise the acid.
There is therefore a desire to produce a process for the production, on a continuous basis, of aldehydes from olefins by hydroformylation using a liquid recycle process under constant conditions chosen by the plant operator for extended, preferably indefinite, periods of time whilst providing maximum utilisation of the catalytic metal and ligand.